"Brief Intervention for Marijuana Use among Latino Youth" is a two-year research project designed to develop and test a school-based, brief intervention for marijuana using, Hispanic/Latino 10th &11th graders. Clinical trials support the effectiveness of brief intervention with adult and older adolescent marijuana users, but these studies have been limited by insufficient ethnic/racial diversity of samples, lack of attention to nontreatment-seeking populations, no direct examination of putative mechanisms of change, and failure to account for possible reactivity-to-assessment effects. 10th &11th grade marijuana users, more than their younger or older counterparts, are at especially high risk for developing problems with marijuana use. Hispanic/Latino adolescents report the highest prevalence of early onset marijuana use, which makes them particularly susceptible to the development of marijuana use problems. The primary goal of the proposed study is to conduct a randomized clinical trial evaluating a school-based motivational interviewing intervention targeting marijuana use among Latino 10th &11th graders. Participants (n = 240) will be randomly assigned to two treatment conditions: (1) brief advice and a personalized feedback report alone (BA+PFR) or (2) brief advice, a personalized feedback report, and motivational interviewing (BA+PFR+MI). Participants will be evaluated at study entry, and at 3- and 6-month follow-ups. At the 6-month followup assessment, 15% of participants will be randomly selected for urinalysis in order to examine the correspondence between self-report and biochemical indices of substance use. We hypothesize adolescents assigned to motivational interviewing will demonstrate significantly greater reductions in marijuana use and marijuana-related negative consequences than adolescents assigned to brief advice plus personalized feedback. Additional aims are to examine (a) mechanisms of change associated with intervention response, (b) how gender, nativity, generational status, and language preference might moderate intervention response. We believe our proposed study is both significant and innovative, and represents an important next step in the development of effective, brief, and school-based interventions for adolescent marijuana users.